SIM900D Hardware Design
M odule
B attery Pack
R
T E M P
T herm istor
T E M P_B A T
V
B A T
V
C H G
5V
Figure12: SIM900D with battery charger and pack connection
Battery temperature measurement is a customization function which is supported by the software in the module.
In above figure, the R
TEMP
is a NTC thermistor, MURATA NCP15XH103F03RC is recommended to use. Its
impedance is 10Kohm at 25 . Pleas
℃
e refer to the above figure for the reference circuit.
3.5.1 Battery pack characteristics
SIM900D has optimized the charging algorithm for the Li-Ion battery that meets the characteristics listed below.
z
The maximum charging voltage of the Li-Ion battery pack is 4.2V and the recommended capacity is
1100mAh. The Battery packs with more than 1100 mAh capacity will cost more time for charging.
z
The pack should have a protection circuit to avoid overcharging, deep discharging and over-current, and the
circuit should be insensitive to pulsed current.
z
The internal resistance of the battery pack including protection circuit should be as low as possible. Its
recommended value does not exceed 150m
Ω
.
z
The battery pack must be protected from reverse pole connection.
It is recommended that the battery pack user integrated into the application is compliant with these specifications.
The battery pack compliant with these specifications is also important for the AT command “AT+CBC”, 1-100
percent of remaining battery capacity returned by “AT+CBC” is related to battery capacity.
On the SIM900D, the build-in circuit of the power management chipset monitors the supply voltage constantly.
Once the Under-voltage is detected, the SIM900D will be powered down automatically.
3.5.2 Recommended battery pack
Following is the specification of recommended battery pack:
SIM900D_Hardware Design_V1.04
2010.10.09
29
